Anesthesia refers to a pharmacologically induced and reversible state of amnesia, analgesia, loss of responsiveness, loss of skeletal muscle reflexes, decreased stress response, or all of these simultaneously. Anesthesia may be administered in a hospital or other medical facility by physicians specializing in anesthesiology, known as anesthesiologists. In the United States, anesthesiologists usually complete four years of undergraduate education, four years of medical school, and four years of postgraduate medical training, or residency. In some cases, anesthesia may be administered by nurse anesthetists or physician assistants, who do not have the same level of training as anesthesiologists. For example, nurse anesthetists and physician assistants may have only completed four years of undergraduate education and two years of graduate training. In many states, nurse anesthetists and physician assistants are required to be supervised by physicians in administering anesthesia.